Income Tax Expense (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended				3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2011 Percent		Sep. 30, 2010 Percent		Jun. 30, 2011 Income Tax Benefit, MPUC Approval of Deferral of PPACA [Member]	Sep. 30, 2011 Income Tax Benefit, MPUC Approval of Deferral of PPACA [Member] Percent	Mar. 31, 2011 DTL Reversal, Revenue Receivable Forgone [Member]	Sep. 30, 2011 DTL Reversal, Revenue Receivable Forgone [Member] Percent	Mar. 31, 2010 DTA Reversal, Elimination of Medicare Part D [Member]
Current Tax Expense (Benefit) [Abstract]
Federal - Current	$ 0	[1]	$ (31.7)	[1]	$ 0	[1]	$ (24.5)	[1]
State - Current	(0.1)	[1]	1.0	[1]	0.1	[1]	0	[1]
Total Current Tax Expense (Benefit)	(0.1)		(30.7)		0.1		(24.5)
Deferred Tax Expense [Abstract]
Federal - Deferred	8.5	[2]	41.0	[2]	19.3	[2]	59.0	[2]
State - Deferred	4.5	[2]	1.2	[2]	6.0	[2]	6.7	[2]
Investment tax Credit Amortization	(0.2)		(0.3)		(0.7)		(0.7)
Total Deferred Tax Expense	12.8		41.9		24.6		65.0
Total Income Tax Expense	12.7		11.2		24.7		40.5
Income Tax Expense, Unusual/Nonrecurring Items [Abstract]
Tax Adjustments, Settlements, and Unusual Provisions									2.9		6.2		4.0
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]
Effective Income Tax Rate, Continuing Operations					24.90%		39.60%
Effective Income Tax Rate Reconciliation, Other Adjustments										2.90%		6.20%
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate					41.00%
Uncertain Tax Positions [Abstract]
Unrecognized Tax Benefits	11.4				11.4
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	$ 0.6				$ 0.6

[1] For the nine months ended September 30, 2011, the federal and state current tax expense (benefit) of zero and ($0.1) million, respectively, (zero and $0.1 million for the quarter ended September 30, 2011) is due to a net operating loss (NOL) which resulted primarily from the bonus depreciation provision of the Small Business Jobs Act of 2010. The 2011 federal and state NOL will be carried forward to offset future taxable income. For the quarter and nine months ended September 30, 2010, we recorded a federal current tax benefit as a result of tax planning initiatives and the bonus depreciation provision in the Small Business Jobs Act of 2010. The 2010 federal NOL was partially utilized by carrying it back against prior years’ income with the remainder carried forward to offset future years’ income.

[2] The nine months ended September 30, 2011, includes a second quarter income tax benefit of $2.9 million related to the MPUC approval of our request to defer the retail portion of the tax charge taken in 2010 as a result of PPACA and a first quarter benefit for the reversal of a $6.2 million deferred tax liability related to a revenue receivable that Minnesota Power agreed to forgo as part of a stipulation and settlement agreement in its 2010 rate case. Included in the nine months ended September 30, 2010, is a first quarter charge of $4.0 million as a result of PPACA. (See Note 6. Regulatory Matters.)